This invention relates to dielectric forming material useful in plasma display panels (PDPs) and, in particular, to dielectric composition used for forming light transparent dielectric layer on a front glass plate of a high strain point in PDPs.
Generally speaking, a PDP has a front glass plate on which a plurality of electrodes are disposed for generating plasma discharge by cooperation with electrodes deposited on a rear glass plate confronting the front glass plate with a gap therebetween. A light transparent dielectric layer is formed on the glass plate to cover the electrodes so as to maintain the plasma discharge generated.
Usually, the front glass plate is made of soda-lime glass or other high strain point glass, while the light transparent dielectric layer is formed from dielectric material comprising low fusion point glass powder, for example, high Pb-content glass powder. When forming the light transparent dielectric layer, the dielectric material is fired or baked at the softening point of the low fusion point glass powder so as to avoid the reaction with metal of the electrodes.
As known in the art, it is important that the dielectric material has various properties such as (1) thermal expansion coefficient compatible with glass plate, (2) firing temperature at 500-600xc2x0 C., (3) excellent defoamability in firing to produce the dielectric layer of high light transmittance and high withstand voltage with a reduced amount of bubbles.
JP-A 11-21148 discloses dielectric material using a glass powder of PbOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94BaO glass which has the thermal expansion coefficient compatible with that of the high strain point glass plate. The PbOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94BaO glass is rapid in viscosity change across the softening point and is, therefore, readily defoamed.
Although the dielectric material using PbOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94BaO glass powder can provide a dielectric layer having a high light transmittance because of its excellent defoamability, it has a problem that the produced dielectric layer has residual large bubbles having diameters of 30 xcexcm (micrometers) or more.
It is an object of this invention to provide a dielectric material which is compatible with the high strain point glass plate in the thermal expansion coefficient, defoamable in firing at a temperature around the softening point, and able to provide a light transparent dielectric layer without large bubbles left therein.
In certain glass such as PbOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94BaO glass described above which is so rapid in viscosity change across the softening point, the present joint inventors found out that bubbles are almost all expelled from the glass at a relatively low temperature during beginning of the firing process but some of them remain as residual bubbles in the glass, the residual bubbles then expanding into large bubbles as elevation of glass temperature in the continuous firing.
In order to improve the PbOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94BaO glass described above, the joint inventors attempted, on one side, to reduce the content of PbO in the PbOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94BaO glass to 24.5 wt. % or less so as to adjust the viscosity change across the softening point relatively slightly slow. On the other hand, it was attempted to increase the BaO content so as to compensate the thermal expansion coefficient lowered due to the reduction of the PbO content.
According to this invention, there is provided a dielectric composition for use in formation of a light transparent dielectric layer in a plasma display panel, comprising glass powder of 90-100 weight % and ceramics powder of 0-10 weight %, said glass powder being powder of glass which consists essentially of, by weight percent, 15-45% BaO, 20-45% ZnO, 15-35% B2O3, 3-15% SiO2, and 0-24.5% PbO.
BaO is an element for adjusting viscosity at a high temperature to affect defoamability of the glass and for elevating the thermal expansion coefficient of the glass. The content of BaO is 15-45%, preferably 20.5-40%, by weight. BaO content less than 15% lowers the defoamability and also lowers the thermal expansion coefficient of the resultant glass to an excessively low level which is not compatible with that of the high strain point glass plate. If BaO content is more than 45%, the resultant glass has an excessively high thermal expansion coefficient which is not compatible with that of the high strain point glass.
ZnO is an element for lowering the softening point and adjusting the thermal expansion coefficient of the glass. The content of ZnO is selected at 20-45% by weight, preferably 22-42%. When the content is selected at less than 20%, the above-described function of ZnO is not achieved. When the content is selected at more than 45%, the thermal expansion coefficient is excessively lowered.
B2O3 is a glass forming element for widening a vitrification range of a composition and should be contained at 15-40% by weight, preferably 16-33%. Less than 15% of B2O3 results in probable devitrification of the glass during the firing. When the content is more than 40%, the glass becomes excessively high in softening point to make it difficult to fire at a temperature of 600xc2x0C. or less.
SiO2 is also a glass forming element and should be selected, in content, at 3-15% by weight, preferably 4-13%. If SiO2 is less than 3%, the resultant glass is readily devitrified during the firing. On the other hand, use of SiO2 more than 15% excessively raises the resultant glass in the softening point to excessively slow the viscosity change across the softening point so that degassing becomes difficult.
PbO is an element for lowering the softening point of the glass and should be selected at contents of 0-24.5% by weight, preferably 0-24%. If the content of PbO is selected at more than 24.5%, the viscosity change of the resultant glass is excessively rapid across the softening point to promote growth of bubbles, this resulting to residual large bubbles remaining in the fired layer.
Contents of PbO, B2O3 and SiO2 in weight should be determined by a ratio of (PbO/(B2O3+SiO2)) less than 1, preferably 0.9. When the ratio is selected at 1 or more, the resultant glass is excessively rapid in the viscosity change across the softening point to promote growth of bubbles. The residual bubbles in the fired layer have possibly a diameter of 30 xcexcm or more.
Further, contents of PbO and BaO in weight should be determined by a ratio of (PbO/BaO)xe2x89xa61.5, preferably 1.3, in order to enable the resultant glass to easily have the thermal expansion coefficient compatible with that of the high strain point glass plate.
It is possible for certain objects to add other ingredients in the glass, for example, CaO and MgO up to 10% in a total amount of them so as to raise the thermal expansion coefficient, and or CuO up to 2% so as to prevent the electrodes of Ag and the transparent dielectric layer from color-changing to yellow as well as to color the electrodes to blue.
According to another aspect of this invention, the glass powder preferably has an average particle size D50 of 3.0 micrometers (xcexcm) or less, and the maximum particle size Dmax of 20 micrometers (xcexcm) or less. If the average particle size and the maximum particle size exceeds the upper limits, there exist large gaps between adjacent glass particles, which promote generation of residual large bubbles in the fired dielectric layer.
The dielectric composition according to the present invention can include ceramics powder such as alumina, zircon, zirconia, and/or titania (titanium oxide) in addition to the glass powder, so as to improve the strength of the fired layer and adjust the appearance thereof. It is preferable that the maximum particle size Dmax of the ceramics powder is 15 xcexcm or less.
In contents, the glass powder and the ceramics powder are 90-100% and 0-10% by weight, respectively. If the ceramic powder content is more than 10%, the resultant dielectric layer fired scatters the visible ray thereby to be opaque.